Effects of silicon on phosphorus fraction dynamics in two tropical soils

Phosphorus (P) availability in tropical soils is often limited due to strong adsorption onto iron (Fe) and aluminum (Al) oxides. Silicon (Si) has been reported to increase P availability, often attributed to competition between Si and P for adsorption sites. However, the mechanisms underlying this interaction remain unclear because most studies have used silicate sources that increase soil pH. This study evaluated the role of Si in the temporal redistribution of soil P fractions in tropical soils. Two contrasting tropical soils (clayey and sandy clay loam) were incubated with four P levels (0, 50, 100, and 200 mg kg −1 ) and four Si levels (0, 240, 480, and 960 mg kg −1 ) using hydrophilic amorphous silica as the Si source, which does not increase soil pH. Soil samples were collected after 15, 30, 90, 180, 270, and 360 days of incubation to determine labile, moderately labile, and non-labile P fractions, as well as soil Si and pH. Phosphate fertilization increased moderately labile inorganic P associated with Fe and Al oxides. Silicon temporarily increased labile P and reduced occluded P during the early incubation period, whereas its effects on P fractions varied with soil texture and incubation time. Overall, Si influenced the redistribution of both inorganic and organic P pools, indicating that its effects on P dynamics in tropical soils are transient and strongly dependent on soil texture and incubation time rather than representing a persistent increase in P availability.